Γ-W1-T5: a real Preserve time-stretcher — write rate is duration, tap rate is pitch

Generalizes the correlation-aligned SOLA delay line so the feed and the shift are
independent rates over one ring. Unity is bit-identical to the shipped read, asserted
against a hash baseline captured pre-change.

src/core/instrument/engine/voice.cpp

@@ -18,7 +18,8 @@ void Voice::presizePreserveShifters(std::int64_t windowFrames) {
     primeBuf_.assign(windowFrames > 1 ? static_cast<std::size_t>(windowFrames) : 0, 0.0f);
 }
 
-void Voice::start(int note, int velocity, const SampleData& sample, bool declickTakeover) {
+void Voice::start(int note, int velocity, const SampleData& sample, bool declickTakeover,
+                  double stretchRate) {
     // Before any state reset, record the pre-cut reference (last rendered output) and mark
     // the compensation pending iff this start is a takeover/steal of a sounding voice and the
     // caller opted in. The ramp is seeded on the first frame rendered after the restart, from
@@ -59,6 +60,9 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     baseRatio_ = keyTrackedRatio(note, sample.rootNote, sample.keyTrack) * velPitchRatio_;
     playMode_ = p.playMode;
     pitchEngine_ = p.pitchEngine;
+    // Clamped once here so the read head's increment and the feed cursor's debt accumulate the
+    // SAME value — they must stay exactly one window apart for the note's whole life.
+    stretchRate_ = instrument::engine::clampStretchRate(stretchRate);
 
     // Clamp into [0, frames): a start at or past the end degrades to 0 (play from the top)
     // rather than starting a voice already off the end.
@@ -234,7 +238,9 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
         }
         // Per-frame feed continues at `p` (the feed bound when the prime exhausted the
         // playable span).
-        feedPos_ = p;
+        stretch_.start(p);
+        shiftL_.setFeedRate(stretchRate_);
+        shiftR_.setFeedRate(stretchRate_);
         if (!loopWrap && primeCount < w) {
             // Sub-window playable span: the source is already exhausted at prime time.
             shiftL_.freezeTail();